The focus of this proposal is to develop human IgG subclass specific bacterial binding proteins. These proteins will be of value for quantifying specific IgG subclasses, studying subclass responses to different antigens and immunization protocols, determining subclass distribution in biological samples and purifying individual human IgG subclasses. The recombinant bacterial IgG subclass reagents developed under this proposal will provide a reliable alternative to existing anti- human IgG subclass monoclonal antibodies which have been shown to have serious limitations in recent studies sponsored by the World Health Organization. During Phase I studies, a recombinant bacterial binding protein specific for human IgG3 was shown to be effective for the sensitive and specific detection/quantitation of human IgG3 in a variety of assay formats. Candidate binding proteins specific for each of the other individual IgG subclasses were also identified. In Phase II, recombinant IgG1, 1gG2 and IgG4 specific human IgG subclass binding proteins will be produced and tested for utility in diagnostic immunochemical procedures and affinity purification methods. Well established genetic engineering techniques will be used to generate recombinant subclass binding proteins with the desired functional characteristics. Phase II studies will provide human IgG subclass specific reagents for use in basic research, immunodiagnostics and bioprocessing. Proposed commercial applications: Bacterial binding proteins that could bind selectively to specific human IgG subclasses will provide key reagents for research and diagnostic immunoassays. Bacterial proteins that could detect IgG subclasses in an analogous fashion to staphylococcal protein A or streptococcal protein G would also have significant commercial applications for large scale purification of IgG subclass antibodies.